Photographic elements relying on silver halide emulsions for image recording have been recognized to possess outstanding sensitivity to light for more than a century. Roentgen discovered X radiation by the inadvertent exposure of a silver halide photographic element. In 1913 the Eastman Kodak Company introduced its first product specifically intended to be exposed by X radiation.
The desirability of limiting patient exposure to high levels of X radiation has been recognized from the inception of medical radiography. In 1918 the Eastman Kodak Company introduced the first medical radiographic product which was dual coated--that is, coated with silver halide emulsion layers on the front and back of the support.
At the same time it was recognized that silver halide emulsions are more responsive to light than to X rays. The Patterson Screen Company in 1918 introduced matched intensifying screens for Kodak's first dual coated (Duplitized.RTM.) radiographic element. An intensifying screen contains a phosphor which absorbs X radiation and emits radiation of a longer wavelength, usually in the near ultraviolet, blue, or green portion of the spectrum.
While the necessity of limiting patient exposure to high levels of X radiation was quickly appreciated, the questioning of patient exposure to even low levels of X radiation has emerged gradually. Although dual coated radiographic elements in combination with intensifying screen pairs have been extensively employed for the less penetrable portions of the body, up until the 1970's direct X ray exposures of radiographic elements without resort to intensifying screens was used for examinations of patient extremities and soft tissue.
Mammography can be used to illustrate the separate development of soft tissue radiography. Mammography employs low energy X radiation--that is, X radiation which is predominantly of an energy level less than 40 keV. Mammographic investigations with silver halide radiographic elements were conducted with direct X ray exposures until the first intensifying screen-silver halide film combination for mammography was introduced in the early 1970's.
Neither dual coated films nor intensifying screen pairs have been used for low energy X radiation mammographic investigations. The reason for this resides in a number of interrelated constraints that are unique to low energy radiography. Perhaps most apparent is that low energy X radiation is limited in its ability to penetrate the phosphors forming intensifying screens. If a conventional thoracic examination intensifying screen pair were used for mammography, only the front screen (the screen nearest the X ray source) would receive any appreciable amount of low energy radiation. Further, being of lower energy, the X rays would be absorbed farther from the silver halide emulsion layer than in thoracic examination, resulting in a less sharp image, whereas mammography requires exceedingly sharp imaging for identifying small and subtle aberrations in tissue.
Nishikawa and Yaffe, "Signal-to-Noise Properties of Mammographic Film-Screen Systems", Med. Phys. 12(1), Jan./Feb. 1985, discloses investigations with a faster film-screen system used in mammography, the 3M XUD/Trimax-2 system: two screens used with a double emulsion anticrossover film. In comparing this symmetrical screen pair and double emulsion film with a rival commercial single screen and single emulsion film combination, Ortho M/Min-R, the symmetrical screen pair was reported to be twice as fast with roughly similar image noise characteristics at low frequencies. No comparison of sharpness was reported.
Haus, "Physical Principles and Radiation Dose in Mammography", Medical Radiography and Photography, Vol. 58, No. 3, pp. 70-83, 1982, published by Eastman Kodak Company, Rochester, N.Y. 14650, provides a review of medical mammography and its development.
Additional items illustrating the state of the art are listed and described in APPENDIX I.